Aetideopsisrostrata is an aetideid calanoidcopepod known after both sexes (female 3.80-4.60 mm and male 3.13-4.20 mm in length). The species is mostly bathypelagic, widespread in Arctic Basin and the Atlantic and Pacific oceans.

Taxonomicdescription:

Female.Total length 3.80-4.46 mm (Tanaka and Omori (1970a) recorded less sizes - 3.01-3.14 mm). Cephalothorax 3-4 times longer than abdomen. Rostrum robust, slightly curved ventrally (lateral view), with robust widely spaced rami. Poorly developed crest arranged at the level of A2 ( dorsal and lateral view). Points of Th5posterior corners reaching or exceeding the midlength of genitalsegment (dorsal view). A1 reaching the midlength of Abd - the end of Abd3. Re2A2 with medialseta. Ri2Md with 10 (8 terminal and 2 posterior) setae. Mx1 and Mx2 typical of the genus. Mxpprotopodite with small projection externally. External spines at Re1 and Re2 exceeding the base of next spine. RiP2segmentation incomplete. Sometimes separation of Ri1 and Ri2P3-P4 incomplete.

Remarks.A.divaricataEsterly, 1911, A.inflataPark, 1978 and A.trichecusVervoort, 1949 considered to be synonyms of A.rostrataSars, 1903 (Markhaseva, 1996). Concerning A.divergens Tanaka, 1957 the opinion of Tanaka and Omori (1970a) is followed in recognizing its identity with A.rostrata.

Male.Total length 3.13-4.20 mm. Cephalothorax 3.1 times longer than abdomen. Rostralrami spaced closer than in female, less robust, sometimes asymmetrical. A1 23-jointed, reaching the end of Th3-Th4. Points of Th5posterior corners exceeding posterior border of genitalsegment (dorsal view). Oral parts in comparison with female rudimental. P1-P4 as in female. P5biramous, asymmetrical, typical of the genus, details of structure varying in different specimens.

Remarks. In comparison with the redescription of male A.rostrata (Shih and Stallard, 1982) in studied specimens there are some distinctions in equipment of Re2A2, Mx1, Md (Markhaseva, 1996).

Verticaldistribution:

The species was found in vertical hauls taken from mesopelagial (Sars, 1903; Jespersen, 1934; Tanaka and Omori, 1970a). In the Arctic Basin found from epi- to bathypelagial, the highest occurrence below 200 m (Markhaseva, 1984); in the Kurile-Kamchatka Trench region usually recorded between 2000 and 4000 m, in haul 2040-3150 from Aleutian Trench region, also in tow 1000-2700 from the Norwegian Sea (Markhaseva, 1996). Recorded in haul from 200 m from the Izu region (Tanaka and Omori, 1970a). Probably interzonal species, primarily bathypelagical, often may be found outside the limits of bathypelagial.

Geographicaldistribution:

Widespread in the ArcticOcean; also in the northern part of Baffin Bay about 75°N (Jespersen, 1934; Shih and Stallard, 1982), in the Barents and the Norwegian seas, the north-eastern Atlantic (Vervoort, 1952a), the southernmost locality here - the North Sea (Rose, 1933), in the south-western part of the AtlanticOcean in the region 56°25' S - 61°51' S (Markhaseva, 1996). In the north-western part of the PacificOcean in the Kurile-Kamchatka Trench region (Markhaseva, 1996) and the Izu region (Tanaka and Omori, 1970a), in the north-eastern part, from the San-Diego region (Esterly (1911) under the name A.divaricata ), and from the region of Aleutian Trench (Markhaseva, 1996), also from the southern part of the PacificOcean (Park, 1978 under the name A.inflata ). The record of A.rostrata from the Mediterranean Sea (Kovalev and Shmeleva, 1982) seems to be doubtful.